Sorry, there should be some documentation.
In your XOOPS admin panel, you should see the SMF Forum icon under the System Admin icon.
When you hold your mouse over it, a popup box will appear with a small menu. Click on the word "Preferences".
Enter the absolute path to SMF in the text box, and save the configuration.
This will tell XOOPS where you installed the SMF forum, so that the module can call information from it.